When used, the term “gas” shall refer to both “gas” and “vapor” in the event the terms are considered to be different.
Control valves are well-known in the art as a means for regulating the rate of flow of a gas through a conduit. One type of control valve is a “flapper” valve (also sometimes referred to as a “throttle” or “butterfly” valve) in which a “flapper” or plate-like valve flow control element is disposed inside a fluid passageway and centrally or close to centrally mounted on a rotatable shaft passing laterally through the interior of the passageway. The orientation of the plane of the flapper is typically adjustable by rotating the shaft in a clockwise and/or counterclockwise direction. The flapper is precisely dimensioned so as to close and more or less seal the passageway to stop or at least substantially reduce fluid flow to a minimum flow when the plane of the flapper is oriented to block the flow of gas, at a 0° position. Alternatively, rotating the shaft and the flapper between 0° and 90° or so, such that the plane of the flapper moves from a fully closed position to a fully opened position, results in the ability to control the flow rate of gas through the passageway by controlling the position of the flapper between some minimum or zero flow to a maximum flow. The simplicity and ease of operation of such flapper valves makes them particularly well suited to regulating the flow of gases in a control system that requires delivery of gases in precise amounts.
Another type of valve useful in regulating gas flow is the pendulum or gate valve. A pendulum or gate valve assembly generally includes a housing containing a pendulum or gate valve flow control element, an interior space and a pair of openings through which gas can enter and exit the interior space.
As its name implies, the valve flow control element (usually referred to as a “gate”) is moveable between fully opened and fully closed positions. The valve flow control element, usually in the form of a disk, is connected to a rotatable shaft by a pivot arm. In a completely opened position the disk of the pendulum valve is typically positioned outside of the flow path defined by the openings so that fluid can enter and exit the interior space of the housing. In a completely closed position the disk is moved into sealing contact with a valve seat surrounding one of the openings so that fluid can not pass through the valve.
The movement of the gate usually requires rotational (i.e., pivotal or lateral) movement between the completely opened position and an intermediate position, and then at least some longitudinal (i.e., translational, linear or axial) movement from the intermediate position to the completely closed position where the gate disk is in sealing contact with the valve seat. In order to obtain this combination of rotational and translational movement, pendulum valves typically use some type of rotation-axial translational mechanism to move the valve body in the desired directions as it moves through its entire range of motion.
Both types of valves, throttle and pendulum valves, can be used to control the rate of flow of gases delivered to processed-controlled systems, such as a CVD (chemical vapor deposition) system. The gases that are used in processes performed by processed-controlled systems are many and varied. A number of important industrial chemicals used in such processes exist in the liquid phase at or about normal room temperature and pressure, but transition to the vapor phase under normal atmospheric pressure at elevated temperatures. For many industrial applications, it is preferred to handle these chemicals in the vapor phase while, at the same time, minimizing excessive, unnecessary inputs of thermal energy. Striking this balance, however, presents special problems in the case of throttle and pendulum valves for regulating the flow of these vapor-phase chemicals. Unless all wetted surfaces of the valve are maintained at temperatures above the liquid-vapor transition temperature of the chemical being regulated, there is a danger of condensation on a valve interior surface resulting in possible corrosion of the valve, contamination of the fluid stream, and pooling of liquid adversely affecting valve operation. As shown in FIGS. 1 and 2, gas flow around the valve flow control element of a flapper valve can condense not only on the valve flow control element as shown at 18, but also on the internal passageway wall of the valve body as shown at 20.
As a result the surfaces of valve flow control element and the valve body can become contaminated with condensate, interfering with the operation of the valve, and shortening its life when the valve needs servicing and/or replacement. Valves are often heated, externally, around the body of the valve. A valve designed so that the valve shaft can be directly heated in an effort to keep the flapper warm is also available. Similarly, over rotation of the flapper in order to wipe the flapper body clean of contaminates is also known.
In the case of the flapper valve, installing a heater in the valve for heating the flapper above the temperature required to keep the process gas flowing through the valve in its gas phase does not always solve the problem. It is expensive, requires power and at times can not completely protect the flapper due to cooling of the flapper by gas flow. There are also cases where the contamination occurs downstream of the leading edge of the flapper due to the fluid dynamics of the gas flow (large pressure drops and potential cooling). An over-rotation method does not necessarily protect all the critical flapper surfaces from contamination.
In the case of both types of valves, it is important to design the valves so that the closed conductance of the valve is at a minimum so as to minimize leakage through the valve. As will be more evident hereinafter, a control gap between the valve flow control element and the valve body results in a relatively large conductance when the valve body is in the fully closed position. Common methods of reducing closed conductance are: creating a small gap (or extending the length of the gap) by a number of mechanical methods including use of soft sealing materials to close the gap. However, creating small gaps by whatever means usually results in more expensive components and mechanisms, with the small gap being susceptible to contamination. If the control gap is filled with a seal of some sort, then wear (which requires periodic replacement), particle generation and poor motion control (due to friction and hysteresis) often result. It is desirable to control and reduce the closed conductance of a control valve without the use of mechanical parts to do so. Further, it is desirable to reduce contamination of a valve flow control element thereby extending the life of the valve before servicing is required.